In the field of electronic system manufacturing, packaging technology has been and will continue to be an important factor to achieve more reliable and less costly packages for denser packaging applications. One of the most common packaging techniques provides for the leads of integrated circuit packages to be soldered into holes in the printed circuit boards, called through-hole packaging. However, the use of surface mount technology to attach and connect integrated circuit components onto printed circuit boards has continued to grow. Surface mount technology is used more widely today in the areas of personal computers, communications and automotive market segments. As is known in the art, surface mountable integrated circuit packages have leads which are folded or bent to allow for their solder connection to a planar surface of a circuit board, eliminating the need for plated-through holes through the circuit board. Surface mount packages can be mounted on both sides of a printed circuit board whereas the through-hole packages can only be mounted on one side, thus significantly reducing the size of the system and increasing the component density of the circuit board. Other advantages of surface mount packages include enhanced electrical performance with the shorter lead lengths and mounting flexibility such as with the gull wing style or the `J` leads, for example, with a small outline integrated circuit package (SOIC) or plastic leaded chip carrier (PLCC). Since the assembled surface mount boards are generally smaller, multilayer printed circuit boards can be used without significantly increasing board cost.
The surface mountable integrated circuit is, however, subjected to greater thermal and mechanical stress during the assembly procedure than are integrated circuits of the dual-in-line (DIP) and similar package types. The mounting of a dual-in-line integrated circuit package to the circuit board is accomplished by wave solder of the underside of the circuit board (i.e., the side opposite that of the integrated circuit), as the pins of the integrated circuit package extend through the plated through-holes in the circuit board. Accordingly, the circuit board itself insulates the integrated circuit package body from the high solder temperatures and harsh chemicals to which the integrated circuit package body is located. However, in the surface mount integrated circuit package body and its contents are directly subjected to the high temperature of the flowing solder, and to harsh chemicals used in the soldering process, such as flux, solder, and cleaning solvents. Conventional surface-mountable integrated circuits that contain only a semiconductor device and its wire connections are generally able to withstand the thermal, chemical and mechanical stresses presented by the surface mount process. As such, the use of surface mount techniques in the manufacture of circuit boards of many types has become widespread, obtaining the density advantages provided by this technology.
Electronic devices which comprise integrated circuit components are usually powered by an external electric supply. It is increasingly more common and desirable to provide electronic components with a back-up power supply to assure retention of stored information in the event of a power loss from the primary supply. In recent years, the use of battery power for many electronic circuit functions has become available, primarily due to advances made in complementary metal-oxide-semiconductor (CMOS) fabrication and design technology. As is well known, CMOS integrated circuits are able to operate with extremely low active power requirements; in the case of CMOS memory devices, such as static random access memories (SRAMs), the power requirements allow operation and data retention in electronic systems to be powered by conventional lithium batteries and other cell types, improving the portability and reliability of modern electronic systems. By way of background, co-pending U.S. patent application Ser. No. 07/995,665, filed Dec. 21, 1992 and entitled "Surface Mountable Integrated Circuit Package with Integrated Battery Mount", assigned to SGS-Thomson Microelectronics, Inc. and incorporated herein by this reference, describes an example of packages containing both a semiconductor integrated circuit and a battery.
Conventional batteries, however, are unable to reliably withstand the temperatures and chemical conditions to which an integrated circuit is subjected during the assembly process. For example, some types of batteries can be irreparably damaged by exposure to temperatures as low as 181.degree. C., which is below the temperature of certain solders used in the mounting process. Other components, such as quartz crystal resonators used in connection with on-chip oscillators, are also vulnerable to these harsh environmental conditions. The harsh chemical environment of the mounting process can also damage the physical construction of these components. By way of further background, copending U.S. patent application Ser. No. 08/225,227, filed Apr. 8, 1993 and entitled "Surface Mountable Integrated Circuit Package with Low-Profile Detachable Module", assigned to SGS-Thomson Microelectronics, Inc. and incorporated herein by this reference, describes an example of a surface mountable chip package and a module containing temperature-sensitive components such as a battery.
Certain system applications install circuit boards with close spacing relative to one another. As a result, the height of components when installed on the circuit board must be kept very low.
It is therefore an object of the present invention to provide a substrate adapted to receive an integrated circuit package and other circuit components such as a battery and an oscillator.
It is a further object of the present invention to provide such a substrate incorporating such components in a low cost and low-profile package.
It is further object of the present invention to provide such a substrate whereby the circuit components can be mounted onto the substrate which is then mounted to the integrated circuit package attached to a circuit board thereby minimizing the need to expose the circuit components to harsh environments during the assembly of the integrated circuit package to the circuit board.
Other objects and advantages will be apparent to those of ordinary skill in the art having reference to this specification in combination with the drawings.